The applicant discovered a new type of post-transcriptional modification of tRNA whereby a base is excised from within the polynucleotide chain of tRNA(Asn) and tRNA(His) and replaced by guanine. These tRNAs have two isoacceptors; one containing G and the other containing a hypermodified base known as Q in the first position of the anticodon. At present, we have preliminary evidence that the role of this enzyme is to remove the guanine from the anticodon of the (G) isoacceptor and replace it with Q or a precursor of Q. Fetal calf serum and amniotic fluid are good sources for the Q or pre-Q that is the substrate of the enzyme and we will purify and identify the substrate from these sources. The guanylating enzyme has been purified to homogeneity in mg quantities. It consists of two subunits with molecular weights of 60,000 and 43,000. We suspect that the large subunit is identical with or closely related to a subunit of His tRNA synthetase reported to be a dimer with a molecular weight of 104,000. A tRNA(His) isoacceptor, tRNA(His), is the best substrate for the guanylating enzyme. We will test to see if this hypothesis is valid. It is not known if higher mammals can synthesize Q or if we are dependent on our intestinal flora for this purine. We want to answer this question using gnotobiotic mice maintained on a Q-free diet of amino acids and yeast extract. The reticulocytes of neonate rabbits do not contain Q in their tRNA; the reason for this is not known and requires an explanation. There are three possible explanations that we will explore: 1) Q is not available because it is not in the diet of preweanling animals; 2) The enzyme that inserts Q or pre-Q into tRNA is absent in the reticulocytes of the neonate animals; or 3) These cells are impermeable to Q or a precursor of Q.